The present invention relates to an automatic gain control (AGC) circuit for controlling the gain of a variable gain amplifier circuit according to the amplitude of an input signal in a communication system or an audio system.
As a conventional AGC circuit, a configuration having an integrator circuit using a capacitor is known. Specifically, such a conventional AGC circuit includes: a variable gain amplifier circuit for amplifying or attenuating an input signal according to the gain controlled with a gain control voltage and outputting the resultant signal; a rectifier circuit for rectifying the output voltage of the variable gain amplifier circuit; an integrator circuit for integrating a voltage rectified by the rectifier circuit into a DC voltage; and a DC amplifier circuit for outputting a voltage proportional to the difference between the DC voltage received from the integrator circuit and a reference voltage as the gain control voltage for the variable gain amplifier circuit. The integrator circuit is essentially composed of a resistor and a capacitor (see U.S. Pat. No. 5,606,284).
In the conventional AGC circuit described above, the signal rectified by the rectifier circuit must be integrated to convert the output signal of the variable gain amplifier circuit to a DC voltage. To accomplish this, the time constant for the integration, determined by the resistance value and the capacitance value of the integrator circuit, must be sufficiently large with respect to the minimum signal period of the analog signal as the input signal of the variable gain amplifier circuit. As a result, the capacitor of the integrator circuit will become too large in capacitance value to be incorporated in an integrated circuit. For example, to secure about 1 ms and about 1 s respectively for the attack time and the recovery time that represent the response time of the AGC circuit, a capacitor having a capacitance value of 0.47 μF is necessary. Such a capacitor can only be provided as an external capacitor.